1. Field of the Invention
This invention relates to a closure for fragile containers and more particularly, to a closure for fragile containers incompletely filled with a gassy liquid.
2. Description of the Prior Art
Modern carbonated beverages containing dissolved carbon dioxide are marketed in sealed fragile containers -- i.e., glass containers, which are substantially but incompletely filled with the gassy beverage and which have a space over the beverage, known as the head space, containing free gas under pressure. The free gas in the head space, typically an admixture of air and carbon dioxide, is a source of danger because, upon fracture of the container, the gas under pressure propels fragments of a broken container at high velocity thereby frequently causing injury to those in the vicinity of the container. In addition, the pressure of the free gas in the head space exerts pressure upon the container cap. This pressure can cause propulsion of the cap at a high speed when the container is stored or initially loosened during unsealing again creating an inherently dangerous situation. Both this phenomenon, known as "cap blow-off" particularly applicable to that portion of the beverage industry using twist-off cap packaging, and the propulsion of fragments upon fracture of the container, present an ever increasing problem to the beverage industry.
The above danger is increased in the summer or in hotter climates due to increased pressure of the free gas in the head space, partly due to the decreased solubility of the carbon dioxide at the elevated temperature. Thus, the danger is greatest when containers filled with carbonated beverages are used in the greatest volume.
A seemingly obvious expedient for avoiding the above problems would be to completely fill the fragile container with liquid thereby eliminating space for collection of pressurized free gas. However, this expedient is not possible since liquid thermal expansion resulting from heating would burst the container. Recognizing the inappropriateness of this technique, the art has focused its attention on three alternative methods to reduce the dangers inherent to container breakage.
The first alternative method is directed solely to encasing the fragile container by coating the entire container surface with a plastic coating. Such a plastic coating, exemplified by Shatterguard and Surlyn, (registered trademarks of Thacher Glass Company and E. I. duPont de Nemours Company, respectively) afford a good surface for handling and to some extent, reduce scratching of the surface of the container, which scratching can lead to fracture. The coating also dampens the explosive velocity of propelled container fragments. However, this method substantially increases the cost of the container, the coating exhibits adverse behavior on return so as to prohibit container refilling and reuse, and the method neglects the danger inherent in the existence of a substantial quantity of pressurized free gas within the head space of the container and hence, does not reduce the danger of cap blow-off.
The second safety method, not in general use, is characterized by a closure means designed to relieve temperature induced free gas pressure buildup inside the head space of the container. Such closures are illustrated by flexible membranes or bellows which inflate outwardly as pressure builds up. This method does not attack the basic safety problem created by the mere existence of a significant quantity of pressurized free gas in a fragile container and does not substantially reduce explosive breakage. Moreover, an inflated and outwardly extending closure creates practical problems in container handling, storage and leakage.
The third and perhaps technically, the most feasible of the prior art methods, comprises a closure means characterized by a combination of a cap and shape retaining volumetric member extending into the interior of the fragile container. The volumetric member is provided with a small orifice in open communication with the interior of the container. Such a closure is described in U.S. Pat. No. 3,733,771, incorporated herein by reference. In use, the fragile container is filled with the gassy liquid and the volumetric member is inserted into the opening of the container. It is of a volume either substantially equivalent to or somewhat less than the volume of the head space within the container such that the volumetric member displaces all or most of the free gas within the head space. The container is then sealed with the cap. Thus most of the gas under pressure in the container is contained within the volumetric member. If the temperature of the gassy liquid increases in sotrage resulting in liberation of carbon dioxide under pressure, the gas thus liberated is forced through the orifice into the volumetric member where it also becomes entrapped within the member. Upon fracture of the fragile container, the gas entrapped within the volumetric member can only slowly leak through the orifice and consequently, the bulk of the free gas is not available to contribute to the explosive force and propel fragments of the container. Hence the explosive potential of the container is substantially reduced, or even entirely eliminated, as little free gas is present in the container outside of the volumetric member, and the gas within the volumetric member is not available to contribute to the explosive potential as it is entrapped within the confines of the volumetric member.
The above closure means overcomes many of the problems of the prior art but is not optimal as it does suffer several limitations. For example, if liquid enters the volumetric member as a result of an increase in liquid volume caused by thermal expansion, or through shaking or inversion of the container, it too becomes entrapped within the volumetric member due to an inability to drain therefrom due, in part, to an unequal pressure between the volumetric member and the interior of the container and in part due to surface tension effects. Due to displacement of liquid from within the container to the volumetric member, there is an increased volume of space within the container in which gas under pressure can collect and a decreased volume of space within the volumetric member wherein free gas can be entrapped. Hence, free gas under pressure is present within the container external to the volumetric member thus again increasing the explosive potential of the container.
The above problem is not overcome by the simple provision of a second orifice in the volumetric member (which would be expected to permit pressure equalization between the interior of the volumetric member and the container) because a meniscus forms between the wall of the volumetric member and the container that prevents pressure equalization, and further, because of surface tension effects, the liquid may not drain adequately through a small orifice.
Further, the volumetric member, though reducing the volume of gas available to propel container fragments upon breakage, and though lessening the danger of cap blow-off upon unsealing of the cap, compared to the current state of the art, does not eliminate the danger of cap blow-off because the gas entrapped within the volumetric member is in direct contact with the cap.